The invention relates to a signature folding device, comprising a saddle-shaped folding sword with an inside guide element on which the signatures are folded. The device furthermore comprises an outside folding element that follows along and operates jointly with the inside guide element for folding the signatures.
Folding devices of the aforementioned type are known. With these devices, the signatures to be folded are guided onto a stationary folding sword and are folded thereon while passing through, with the aid of the outside folding element that follows. Belts, round strings, V-belts or chains, for example, can form the folding element that follows. A folding design of this type has the particular disadvantage that the folding occurs only gradually and that the signatures do not fit form-fittingly against the folding sword until the end of the folding operation. Only at the end of this folding operation are the signatures held securely in place and can thus be transported without slipping. Prior to this, the signatures are quite unstable and can deviate from the predetermined course, thus resulting in an inexact folding. Another difficulty is that the signatures to be folded are slowed down on the inside, by the folding sword that is stationary, and at best have a tendency toward smearing. The higher the effect of the outside folding element, the higher the tendency to smearing.
The folding element on the inside is, for example, a chain with fitted-on roof-shaped elements or is formed by round strings.
A folding device as defined in the preamble is disclosed in Swiss Patent document CH 615 646 A. The inside folding element described in this document is formed by a linear row of folding rollers. On the one hand, the folding element on the inside should be a precision element, so that a sharp, closed and if possible complete fold can be formed. On the other hand, this folding element should also be robust to prevent the element from being bent or damaged during backups, which requires a certain structural size. These two requirements conflict with each other, so that compromises are necessary in the one or the other direction. If the folding element on the inside consists of rollers, arranged one behind the other as specified in the Swiss Patent, the difficulty arises that during the folding process the front edges of the signatures to be folded must repeatedly run in via the subsequent roller. If the signatures to be folded are not always positioned exactly the same way for the run in, the resulting fold is not sharp and the folding accuracy cannot meet high requirements. A precise run in of the signatures onto the rollers is difficult to realize since stresses in the signatures to be folded are hard to avoided during the process of folding the signatures while they are passing through.